This invention pertains generally to methods and apparatus for treating tissue and, more particularly, to methods and apparatus for treating tissue utilizing radio frequency energy.
Medical devices have been provided for treating tissue of a mammalian body by the use of radio frequency energy. See, for example, U.S. Pat. Nos. 5,370,675, 5,385,544 and 5,549,644. Radio frequency energy passing from an electrode of such a device through the adjoining tissue causes heating of the tissue. In a monopolar device, the radio frequency energy passes from the active electrode to an indifferent electrode typically in contact with the exterior of the body of the patient. In order to cause desired tissue ablation and subsequent necrosis, the treated tissue is heated to a temperature in excess of approximately 47xc2x0 C. However, if the temperature of the tissue being treated is elevated too high, dehydration and later charring of the tissue can occur. Such dehydration and charring can increase the impedance of the tissue to a level that prohibits radio frequency from traveling through the tissue. In view of the foregoing, prior systems have monitored the impedance of the active electrode circuit and adjusted the amount of radio frequency energy supplied to the electrode in response to such impedance measurements.
Some previously provided medical devices utilize multiple radio frequency electrodes that can be disposed in the vicinity of each other in the tissue being treated. Where the amount of radio frequency energy being delivered to such electrodes varies, it has been found that crosstalk can occur between the electrodes. In a monopolar device, for example, current may travel from one active electrode down the circuit of the other active electrode to the radio frequency generator. Where the impedance of the active electrode circuit is being monitored, such crosstalk current can result in erroneously high impedance measurements and thus unwanted reductions in radio frequency energy to the active electrode with the high impedance measurement.
It would be desirable to provide a method and apparatus that reduces the contribution of crosstalk between adjacent electrode circuits when monitoring the impedance of the electrode circuits.
A method is provided for calculating the impedance in a first circuit coupling a first radio frequency electrode to a radio frequency controller where the first radio frequency electrode is disposed in tissue of a mammalian body in the vicinity of a second radio frequency electrode coupled to a second circuit distinct from the first circuit. In the method, the impedance of the first circuit is measured to determine an impedance value IMP when radio frequency energy is being supplied to the first and second radio frequency electrodes. The impedance of the first circuit is also measured to determine an impedance value IMP0 when radio frequency energy is being supplied to the first radio frequency electrode but not to the second radio frequency electrode. The impedance value IMP0 is subtracted from the impedance value IMP to determine an impedance delta value IMPxcex94. The impedance of the first circuit is remeasured to determine an impedance value IMP2 when radio frequency energy is being supplied to the first and second radio frequency electrodes. The impedance delta value IMPxcex94 is subtracted from the impedance value IMP2 to arrive at a calculated impedance value for the first circuit. A computer-readable memory and apparatus utilizing the method are provided.